An immunoradiometric assay for endopeptidase-24.11, which depended on the absorption by tissues of a monoclonal antibody, GK7C2, was established. The optimum conditions for the assay were defined and its correlation with an enzymic assay determined. The immunoassay was used to survey the endopeptidase in crude homogenates of various tissues of the pig. Detergent treatment decreased the sensitivity of the assay but did not invalidate it. Although the endopeptidase was found in many tissues, it was neither uniformly nor ubiquitously distributed. Kidney cortex was confirmed as the major location of the endopeptidase, containing 5000 ng/mg of protein. Lymph nodes were also very active (1370 ng/mg), followed by chondrocytes from articular cartilage (650 ng/mg). In the gut, the endopeptidase was concentrated mainly in the jejunum (130 ng/mg). Various glands (salivary, adrenal, anterior pituitary and pancreas) also contained the antigen in the range 20-55 ng/mg of protein. Lung contained only 5 ng/mg of protein and, in other tissues examined, little or none was detectable. In particular, other lymphoid tissues (spleen, thymus, tonsillar tissues) were relatively poor sources, and none was detectable in peripheral-blood leucocytes or in peritoneal macrophages.

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